The present invention is related to a cage of a rolling bearing, specifically to a rolling bearing which is low in non-repeatable runout and proper as the bearing for the spindle of a hard disc (HDD), a machine tool shaft, etc.
Rotation in a rotating device using the bearing causes more or less vibrations. The conditions of vibration required to the rotating device are becoming more severe year by year. For example, the spindle motor for driving an HDD is required to be highly precise in non-repeatable runout (vibration which does not synchronize with the rotation), in a degree of less than 0.1 xcexcm. The rotation precision of the shaft in a precision lathe, the size of which is larger than the motor for driving the HDD, is required to be substantially the same as in the motor for driving the HDD.
In the case of the machine tool shaft, there is a problem of the non-repeatable runout as in the motor for driving the HDD. Specifically, with the repeatable runout (vibration which synchronizes with the rotation), the processed surface is continuous and clean, but in the case of non-repeatable runout, the processed surface is rough with undulation, which reduces the quality of the processed surface.
The dominant vibration of the non-repeatable runout has a frequency fc (rotation cycle of the cage). This vibration (hereinafter referred to as xe2x80x9cfc runoutxe2x80x9d) occurs for example because there is a difference in diameter between the rolling elements within the cage, or because with respect to the arrangement space, the rolling elements in the bearing are displaced from uniform space arrangement during rotation.
If the rolling elements are not placed in a uniform space arrangement in the bearing during rotation, the fc runout (defined above) is caused due to the mechanism as follows. Specifically, under resilient contact between the preloaded rolling elements and the inner and outer races, a component of force is produced in the respective rolling elements to radially push the inner race. Then, if the rolling elements are not placed in a uniform space arrangement in the bearing during rotation, the vector sum (xcexa3F) of these components of force is not zero. As a result, the axis position is changed during rotation to produce the fc runout.
With the recent improvement of process technology, it becomes possible for example to process rolling elements in the precision of several tens of nanometers (nm). Consequently, the deviation from the spherical form of balls for the rolling elements is high, so that the differences in diameter between the rolling elements are very small.
Moreover, in the case of a cage of the inner race guide type, if the weight of the cage is not uniform in the circumferential direction, the shaft is subjected to rotation load due to the non-uniformity of weight. This also causes fc runout, which, however, can be reduced sufficiently by making the cage from a light material such as synthetic resin.
Accordingly, recently, the main reason of fc runout is the displacement of rolling elements from the uniform space arrangement in the bearing during rotation. It should be noted however that even if the cage is produced with a precisely uniform arrangement space for the pockets, once the cage is radially moved in the bearing, with respect to the arrangement space, the rolling elements may be displaced from the uniform space arrangement in the bearing during rotation.
Accordingly, conventionally, not only the arrangement space of rolling elements is precisely uniform, but also the guide gap is small in the cage, so that the radial movement of the cage is kept small to achieve a uniform space arrangement in the rolling elements in the bearing during rotation.
However, in the prior art technology as mentioned above, the cage is radially movable in the bearing by a small limited amount, and therefore inconvenience such as seizure is caused particularly at high speed rotation, which is a problem.
An objective of the present invention is to provide a cage taking the problem in the prior art technology into consideration with the guide gap being sufficiently secured, when the cage is radially moved in the bearing during rotation, with respect to the arrangement space, the rolling elements in the bearing are substantially in the uniform space arrangement.